Field of the Disclosure
The present disclosure relates generally to wireless communication systems and, more particularly, to transmissions in unlicensed frequency bands of wireless communication systems
Description of the Related Art
Unlicensed frequency bands are portions of the radiofrequency spectrum that do not require a license for use and may therefore be used by any device to transmit or receive radiofrequency signals. For example, the Unlicensed National Information Infrastructure (UNII) is formed of portions of the radio spectrum that include frequency bands in the range of 5.15 GHz to 5.825 GHz. For another example, the industrial, scientific, and medical (ISM) radio bands are portions of the radio spectrum that are reserved internationally for unlicensed communication. The ISM radio bands include bands with a center frequency of 2.4 GHz and a bandwidth of 100 MHz, a center frequency of 5.8 GHz and a bandwidth of 150 MHz, and a center frequency of 24.125 GHz and a bandwidth of 250 MHz, among other frequency bands. Unlicensed frequency bands can be contrasted to licensed frequency bands that are licensed to a particular service provider and may only be used for wireless communication that is authorized by the service provider.
Wireless communication devices that transmit or receive signals in licensed or unlicensed frequency bands are typically referred to as nodes, which may include Wi-Fi access points that operate according to IEEE 802.11 standards in the unlicensed spectrum or base stations that operate in licensed spectrum according to standards such as Long Term Evolution (LTE) standards defined by the Third Generation Partnership Project (3GPP). Base stations that operate according to LTE can implement supplementary downlink (SDL) channels in the unlicensed spectrum to provide additional bandwidth for downlink communications to user equipment that are also communicating with the base station using channels in a licensed frequency band. The licensed frequency bands may be referred to as LTE-L bands and the unlicensed frequency bands may be referred to as LTE-U bands. In some cases, base stations may also support uplink communication in the unlicensed spectrum.
Nodes that share the unlicensed frequency band should coordinate their transmissions so that the available resources of the unlicensed frequency band are fairly shared among the different nodes. For example, base stations typically perform carrier sensing to select channels for downlink transmission in unlicensed frequency bands. For example, a base station may measure energy received in channels in the unlicensed frequency bands to identify a “clear” channel. A channel may be considered clear an average of the received energy from other LTE base stations or Wi-Fi access points on the channel is below a threshold value. The base station may then use the clean channel for downlink transmissions. If the base station is unable to identify a clean channel, the base station has to share the channel with one or more other transmitting nodes.
In countries such as the U.S. that do not mandate Listen-Before-Talk (LBT) regulations for operation in the unlicensed spectrum, the LTE base-station may employ a transmission cycle with appropriate duty cycle for ON/OFF period to co-exist on shared channels with Wi-Fi and other LTE base-stations on the same channel of the unlicensed spectrum. An LTE base station may share a channel in the unlicensed frequency band with one or more access points by transmitting signals on the channel for a predetermined time interval (ON) and bypassing transmissions during a subsequent time interval (OFF). For example, the ON interval is typically a multiple of 40 milliseconds (ms) so the ON interval may be 40 ms, 80 ms, 120 ms, or longer. The duty cycle for the ON/OFF time intervals is determined by the number of devices that are sharing the channel. For example, the base station may transmit during a 40 ms ON interval and then bypass transmissions for an 80 ms OFF interval if it is sharing the channel with two Wi-Fi access points.
The extended ON interval used by LTE base stations can cause performance degradation for delay sensitive Wi-Fi applications. For example, voice applications may suffer a noticeable reduction in sound quality if the corresponding Wi-Fi application is unable to transmit during a periodically repeating 40 ms time interval. The performance degradation increases if the ON interval is increased to 80 ms, 120 ms, or longer. Wireless communication systems that implement Listen-Before-Talk (LBT) protocols address this issue by requiring transmitting nodes to listen for a free channel before acquiring the channel for transmission. The maximum channel occupancy is limited to 4 ms in Japan and 10 ms in Europe, so that delay sensitive applications do not have to wait for extended time intervals before acquiring a free channel. However, LBT standards have not been implemented in all countries and implementing the LBT standards will require changes to the existing LTE waveform, which may delay adoption of the LBT standards for a few years.